Excellent link, all sorts of nifty rare stuff. Thanks.
Excellent link, all sorts of nifty rare stuff. Thanks.
skiingman, it seems we are in agreement 
One more thing worth mentioning is that the reason (among others) that you lose grip on the axle that has more load transfer is the load sensitivity of the tyre:
The grip of the outside tyre increases with increasing load and the grip of the inside tyre decreases with decreasing load.The rate of increase/decrease isn't the same in both cases though. The higher the load on the tyre, the lower the increase rate. This means that the overal grip of the axle drops with load transfer.
In one sentence: when increasing load transfer from inside to outside, the inside tyre loses more grip than the outside gains:
And nice link Vykos.
One more thing worth mentioning is that the reason (among others) that you lose grip on the axle that has more load transfer is the load sensitivity of the tyre:
The grip of the outside tyre increases with increasing load and the grip of the inside tyre decreases with decreasing load.The rate of increase/decrease isn't the same in both cases though. The higher the load on the tyre, the lower the increase rate. This means that the overal grip of the axle drops with load transfer.
In one sentence: when increasing load transfer from inside to outside, the inside tyre loses more grip than the outside gains:
And nice link Vykos.
yeah, I said:
"Tire adhesion available is not linearly related to the vertical component of force on the tire (twice the force doesn't result in twice the grip), so less weight transfer is always better."
which is not nearly as thorough and well defined as your explanation above.
That link really is cool, I had some fun reading about that RFH LeMans hydraulic suspension.
All true. It's worth noting that, for my South City setups, I run softer ARBs, preciesly because the track is bumpier, including some very noticable bumps from manhole covers and the roughness of the brick-road chicane. And for my RallyX setups I run very soft ARBs.
Well, the main point in favor of live axles besides durability is that, with a live axle, when one wheel is pushed upwards by a bump, it exerts a downward force on the other wheel as the axle rotates around its central point. Ignoring the effects of non-flat tire contact, which is valid with very soft ground, large rocks, or extremely low tire pressures (all hallmarks of offroading), it's easy to see that more downward force means more grip. More grip is always very desirable.
An independent suspension with cross-linked pneumatic or hydraulic springs will have the same effect, and this system has been used to good effect on some Land Rovers, but the system (at least on Land Rovers) is also famous for being very unreliable and hard to repair. It's also undrivable on normal terrain without switching back to conventional springing. You're probably right about it being better in the case of unlimited funds, but rockcrawling isn't yet such a big sport that most people have that kind of money and engineering skill.
Well, there's always the matter of personal taste. I can certainly see that softer ARBs on a formula-style car would certainly be no great disadvantage, and the advantages may be enough to justify it. In a perfect world, of course, the CoG and the roll center would coincide, and then there would be no body roll at all, but nobody has ever made a car that had a CoG that low. I still think that in the absence of high-tech systems (pneumatics, Bose active suspension, etc.), ARBs are essential for high-centered cars like typical road cars.
Isn't this a great example of bad ARBs? Looks like the rear ones are too soft and allow 'big' rolling, while the front ARB is too stiff and the front axis can't roll enough to touch the ground with both wheels.
This is both an example and a test wether I understood it all right.
Vain
This is both an example and a test wether I understood it all right.
Vain
Could also be a problem of frame stiffness...
It's a Porsche, I'd say it's not a good setup, but given RWD, heavy in the back, and powerful, it doesn't really shock me. Accelerating out of a corner with a lot of grip could do that.
Oh, and Vain, you're right. Soft rear ARB + stiff front ARB could very well cause what that picture shows. Assuming the suspension has been modified to act that way, the guy is trying to compensate (overcompensate?) for the Porsche's tendency to oversteer.
Yeah I agree totally. That kind of setup is popular on front engined sedans in order to minimize power on oversteer to get good grip on the way out of the corners. On something with the engine behind the rear axle, its only more necessary.
I think if you look closely at that pic you'll see the thing really is all twisted out of shape. Look at the plane of the rear spoiler and that of the front bumper. :O Could probably use better a beefier cage, but stiff isn't always fast and that setup looks rather vintage anywho.
Hehe, i went to get my European car magazines, there was a series of
articles called Suspension Guide (not a buyers guide..) a few years back.
December 1999 contains Part 2 - Controlling body roll. I found the articles
very informative and pleasantly objective including both scientific and
vulgarised explanations. It is in one of those articles that i learned about the
relation between load and grip on a tire. That alone seems to be the main
idea behind most suspension tuning. Like stated earlier, as the load
on a tire increases, it's grip also increases, but up to a certain point where
grip falls down again. It is not a linear relationship, that means there's an
optimal operating zone, and therefore there are also overloaded or unloaded
situations, both reducing grip until there is virtually none left.
I find most of the disagreement is on the wording more than the actual
understanding of the situation. One thing i find very confusing in your posts
is the liberal use and exchange of COG and ROLL AXIS (a result of the
ROLL CENTERs, also referred to as 'centroids' in some posts), although
i'm sure you guys know the difference, i think it's important to mention to
others that they are 2 different things and 'roll' is actually the result of their
interaction.
There are 2 ROLL CENTERs, one for the front and one for the rear, each a
result of the suspension geometry (the front of the car rolls around a
specific axis while the rear rolls around another axis).
The ROLL AXIS is the line (axis) formed between the front and rear ROLL
CENTERs. In short, the car body rolls on an axis that is defined by a line
from the front roll center to the rear roll center.
The COG is the FORCE acting around that ROLL AXIS, which is the line
between the front and rear ROLL CENTERs.
This is the actual rolling force.
It can be easier to simply think of this as the car's weight. Consequently,
it's location is basically an average of the car's parts/weights, a more
appropriate name would be 'center of mass'. I don't think there's a way
to calculate gravity's exact position even less it's 'center'
Anyone who had physics will tell you that Force(N) = Mass(kg) * Acceleration
(ms²). So 'weight' is a FORCE and not a MASS, as the common misconception
goes, your MASS is the same on the moon as it is on earth. What DID change,
is the FORCE you exert on the ground, this is what we call weight.
Something else i want to mention is that all this is not about weight transfer
as much as it's about contact patch management imo, for the reasons
specified above, namely the load/grip relationship of a tire, what we want is
not to reduce body roll as much as we want to even out this 'weight' (force)
over the 4 tires as not to overload any. The 'reduced' weight transfer is only
another way of saying that the COG moves less. Reduced body roll being the
visible indicator. This (COG moving less) is good for 2 reasons:
1- A moving COG creates torque around the ROLL AXIS which is what we
notice as body roll. This torque is the actual 'weight shifting'. The result is
an increase in the outer tire's load and a decrease of the inner ones. Again,
in theory, this isn't a problem unless the outer tire are OVERloaded.
2- Supposing that the COG is perfectly along the center of the car, which it
usually is close to, as the body leans, the COG becomes off center and tends
to 'fall to the ground', this actually ADDS itself to the centripede force, the
one that 'pushes you sideways'). Think of an object on top of a ball, if you
put it perfectly on the center, it will stay there, relatively static. If however
you place it slightly off-center, the object will fall off. Gravity is the force
that was at work there and it also applies to the COG . Obviously, the COG
would be the object and the ball would be the ROLL AXIS.
I still like to think that equalising the tire load is the actual goal. I just find
it simpler to understand the effects on handling. A leaning body doesn't
slide, a fully deformed and overloaded contact patch does. There is nothing
dramatic about tranferring weight in itself if it wasn't that our current tires
do NOT like to be overloaded.
To close the loop, anti-roll bars work to reduce the COG's movement (in
amplitude, not in acceleration though) and therefore optimizes the tire's
contact patches by spreading the load on all 4 tires to prevent overloading
them. At the extreme, if you overload all 4 tires, no ARB will help you, oh
and you're a n00b, learn to drive I'm simplifying by saying all 4 wheels,
but it's actually 2 bars each optimising grip on 2 tires. This is why ARBs
are also usefull for tuning as it effectively changes the front/rear 'grip'
bias. This is where the light starts dimming....
I told myself to keep it short, but i guess i just can't help myself ...
articles called Suspension Guide (not a buyers guide..) a few years back.
December 1999 contains Part 2 - Controlling body roll. I found the articles
very informative and pleasantly objective including both scientific and
vulgarised explanations. It is in one of those articles that i learned about the
relation between load and grip on a tire. That alone seems to be the main
idea behind most suspension tuning. Like stated earlier, as the load
on a tire increases, it's grip also increases, but up to a certain point where
grip falls down again. It is not a linear relationship, that means there's an
optimal operating zone, and therefore there are also overloaded or unloaded
situations, both reducing grip until there is virtually none left.
I find most of the disagreement is on the wording more than the actual
understanding of the situation. One thing i find very confusing in your posts
is the liberal use and exchange of COG and ROLL AXIS (a result of the
ROLL CENTERs, also referred to as 'centroids' in some posts), although
i'm sure you guys know the difference, i think it's important to mention to
others that they are 2 different things and 'roll' is actually the result of their
interaction.
There are 2 ROLL CENTERs, one for the front and one for the rear, each a
result of the suspension geometry (the front of the car rolls around a
specific axis while the rear rolls around another axis).
The ROLL AXIS is the line (axis) formed between the front and rear ROLL
CENTERs. In short, the car body rolls on an axis that is defined by a line
from the front roll center to the rear roll center.
The COG is the FORCE acting around that ROLL AXIS, which is the line
between the front and rear ROLL CENTERs.
This is the actual rolling force.It can be easier to simply think of this as the car's weight. Consequently,
it's location is basically an average of the car's parts/weights, a more
appropriate name would be 'center of mass'. I don't think there's a way
to calculate gravity's exact position even less it's 'center'
Anyone who had physics will tell you that Force(N) = Mass(kg) * Acceleration
(ms²). So 'weight' is a FORCE and not a MASS, as the common misconception
goes, your MASS is the same on the moon as it is on earth. What DID change,
is the FORCE you exert on the ground, this is what we call weight.
Something else i want to mention is that all this is not about weight transfer
as much as it's about contact patch management imo, for the reasons
specified above, namely the load/grip relationship of a tire, what we want is
not to reduce body roll as much as we want to even out this 'weight' (force)
over the 4 tires as not to overload any. The 'reduced' weight transfer is only
another way of saying that the COG moves less. Reduced body roll being the
visible indicator. This (COG moving less) is good for 2 reasons:
1- A moving COG creates torque around the ROLL AXIS which is what we
notice as body roll. This torque is the actual 'weight shifting'. The result is
an increase in the outer tire's load and a decrease of the inner ones. Again,
in theory, this isn't a problem unless the outer tire are OVERloaded.
2- Supposing that the COG is perfectly along the center of the car, which it
usually is close to, as the body leans, the COG becomes off center and tends
to 'fall to the ground', this actually ADDS itself to the centripede force, the
one that 'pushes you sideways'). Think of an object on top of a ball, if you
put it perfectly on the center, it will stay there, relatively static. If however
you place it slightly off-center, the object will fall off. Gravity is the force
that was at work there and it also applies to the COG . Obviously, the COG
would be the object and the ball would be the ROLL AXIS.
I still like to think that equalising the tire load is the actual goal. I just find
it simpler to understand the effects on handling. A leaning body doesn't
slide, a fully deformed and overloaded contact patch does. There is nothing
dramatic about tranferring weight in itself if it wasn't that our current tires
do NOT like to be overloaded.
To close the loop, anti-roll bars work to reduce the COG's movement (in
amplitude, not in acceleration though) and therefore optimizes the tire's
contact patches by spreading the load on all 4 tires to prevent overloading
them. At the extreme, if you overload all 4 tires, no ARB will help you, oh
and you're a n00b, learn to drive
I'm simplifying by saying all 4 wheels,but it's actually 2 bars each optimising grip on 2 tires. This is why ARBs
are also usefull for tuning as it effectively changes the front/rear 'grip'
bias. This is where the light starts dimming....
I told myself to keep it short, but i guess i just can't help myself ...
From Carroll Smith's Tune To Win:
"The greater the resistance of the springs, the less roll will result--but there will be no significant effect on the amount of lateral load transfer because the roll couple has not been changed and there is no physical connection between the springs on opposite sides of the car. The same cannot be said of the resistance of the anti-roll bars. In this case, because the bar is a direct physical connection between the outside wheel and the inside wheel, increasing the stiffness of the anti-roll bar will both decrease roll angle and increase lateral load transfer." 38.
I've no idea why I just quoted that.
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Nature of anti-roll bars
(35 posts, started )
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